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MEEA Lumber Spine | Feature
19 shows the reduced horizontal acceleration of the surrogate magazine mass for the taped condition when compared with the heavy weight condition.
DISCUSSION
Effect of lumbar spine assemblies and their resultant ATD sitting postures
The FAA Hybrid III and the Hybrid III with the pedestrian spine showed similar results in peak vertical lumbar load and peak vertical pelvis acceleration when compared with each other. It should be noted that, in the first test, the rigid pelvis pierced the pelvis skin. Throughout the subsequent tests, the same pelvis was used, producing consistent results. The mean peak vertical lumbar loads measured for the two straight spines
varied by 10% for the higher acceleration pulse and only 3% for the lower acceleration pulse. This is a consequence of the straight spines being similar in design and therefore producing a very similar posture. However, it is important to note that these acceleration pulses produced peak vertical lumbar loads that are well in excess of the threshold of
6.7 kN recommended in FAA guidelines [13]. Considering the difference in peak vertical lumbar load was 10% for values around 15 kN and 3% for values around 12 kN, it is unlikely that the percentage difference for values around the FAA threshold of 6.7 kN would be more than 10%. Therefore, the peak vertical lumbar loads at the two acceleration pulses measured between the two sitting postures induced by the straight spine should remain similar, regardless of the acceleration pulse used, when seated on a rigid seat. These tests on a drop tower to simulate a blast event were done with the distance between the ATD H-point and the back of the seat minimized and the ATD torso oriented as upright as possible. In such a scenario, these results demonstrate that the lumbar spine assembly and the sitting posture for the straight lumbar spine assemblies appear to not have a large effect on the ATD response.
The Hybrid III with the curved spine generated the highest peak vertical lumbar load, 13% greater than the lowest generated peak vertical lumbar load produced by the FAA Hybrid III. However, due to the angle of orientation of the lumbar load cell, the measured lumbar load of the Hybrid III with the curved spine is at an angle of 22∞ to the vertical and therefore it may be the reason why the lumbar load measured is greater than the straight lumbar spine
assemblies. The Hybrid III with the curved spine generated the lowest peak vertical pelvis acceleration as a result of the pelvis being pushed further forward, due to the curvature of the spine. Consequently, the measured peak horizontal pelvis acceleration was
larger than those associated with the straight lumbar spine assemblies.
Effect of Equipment mass
With the addition of body-borne equipment mass, an increase in peak loading might seem likely, yet the results demonstrated very little difference in peak vertical pelvis acceleration and peak vertical lumbar load. The additional mass does not appear to influence the loading in the time frame relevant to peak loading. This is because the BBE mass
does not create any substantial load on the body until after the peak loading event, as evidenced by the accelerometer mounted to the magazine mass. Figure 13 demonstrates
that the magazine mass does not provide any vertical acceleration until approximately 7 ms, with the peak vertical acceleration occurring at approximately 15 – 20 ms. While the BBE does not affect the ATD response during the peak loading event, there are minor differences in the time history graph of the pelvis acceleration between the ATD without BBE and with BBE at approximately 12 ms. This is the time where the magazine mass is accelerating in the vertical direction.
Placing the 7.2 kg mass on top of the
lumbar spine load cell demonstrated that equipment will have an influence on loading even during a short time frame less than 10 ms. However, in a real testing situation, the equipment is not placed underneath the skin. When the 7.2 kg equipment mass is taped
to the body with a tighter attachment, the lumbar load increases by almost 800 N when compared to the heavy BBE mass condition. The movement in the horizontal direction
is less than when using the Modular Light- Weight Load-Carrying Equipment (MOLLE), i.e. what military personnel actually wear. This illustrates that the transfer between the equipment and the lumbar spine is heavily affected by how the equipment is attached to the body. Aggromito et al [14] used a mass- spring damper mechanism to show that the equipment attachment stiffness is important to the loadings on the body, albeit during a simulated helicopter crash.
SUMMARY/CONCLUSIONS
The experimental study conducted an in-depth experimental plan to determine the effect of equipment mass and lumbar spine assembly on the effect of injury. Overall 32 tests were completed on three ATDs. It was found that the FAA and Pedestrian straight lumbar spine assemblies generated similar ATD responses in drop tower tests using a rigid seat. In contrast, the curved lumbar spine assembly generated a lower pelvis acceleration and a higher lumbar load than the straight lumbar spine assemblies. BBE mass will not affect the peak ATD responses when the ATD is seated on a rigid seat during a drop test
due to the short duration of the impact.
The additional experimental conditions demonstrated that equipment attachment to the body can influence the loading on the ATD. When the equipment was taped, to the jacket skin of the body, an increase in lumbar load was measured. The results demonstrate that any straight lumbar spine assembly can be used, as they offer minimal variation between results in key injury criteria.
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